Kokuji

This is because the phonetic element of phono-semantic kanji is always based on the on reading, which most kokuji don't have, leaving semantic compounding as the only alternative.

Kokuji are especially common for describing species of flora and fauna including a very large number of fish such as 鰯 (sardine), 鱈 (codfish), 鮴 (seaperch), and 鱚 (sillago), and trees such as 樫 (evergreen oak), 椙 (Japanese cedar), 椛 (birch, maple) and 柾 (spindle tree).

[1] The term kokuji in Japanese can refer to any character created outside of China, including Korean gukja (國字, 국자) and Vietnamese chữ Nôm.

Historically, some kokuji date back to very early Japanese writing, being found in the Man'yōshū, for example—鰯 iwashi "sardine" dates to the Nara period (8th century)—while they have continued to be created as late as the late 19th century, when a number of characters were coined in the Meiji era for new scientific concepts.

However, SI units in Japanese today are almost exclusively written using rōmaji or katakana such as キロメートル or ㌖ for km, キロリットル for kl, and キログラム or ㌕ for kg.

[note 1] Another example is 搾, which is sometimes not considered kokuji due to its earlier presence as a corruption of Chinese 榨.